An offline Nessus instance cannot pull security checks automatically. You must manually feed it the downloaded plugin package.
After installation, start the Nessus service: nessus pro offline activation
On the offline machine’s Nessus web interface: An offline Nessus instance cannot pull security checks
Nessus Professional requires an offline activation process when deployed on isolated, secure networks without direct internet access. Performing a Tenable Nessus Professional offline registration ensures your vulnerability scanner remains functional while adhering to strict air-gapped security policies. Prerequisites Before Activation nessus pro offline activation
nessuscli.exe fetch --register-offline nessus.license